Solder wafer bumps conventionally comprise solders of the tin-lead (Sn—Pb) alloy group. The Sn—Pb solders may be formed in a variety of compositions including the low melting eutectic comprising 63% Sn and 37% lead. High Sn alloys are called fine solders and are used extensively in electrical work. Many Sn—Pb alloy compositions exhibit broad pasty temperature ranges which enhance their workability.
Recent regulatory and environmental developments have increased interest in Pb-free solders. Accordingly, pure Sn, Sn—Cu, Sn—Bi, Sn—Ag, and ternary Sn alloys have been explored as potential alternatives to Sn—Pb alloys. Of particular interest are the Sn—Ag alloys because of their performance advantages such as low resistivity, stability, the ability to achieve a wide range of melting points, and the elimination of alpha particle emissions by using pure Sn sources.
A particular problem associated with the use of Ag in Sn—Ag alloy solder wafer bumping is associated with the spontaneous reduction of Ag ions from the electroplating bath. For example, Ag ions, being very noble, have a tendency toward immersion/displacement plating upon exposure to certain UBM layers, particularly Cu layers. Accordingly, precise control of the Ag ion concentration in the electroplating solution and therefore control of the Ag metal content and uniformity in the Sn—Ag solder wafer bump is rendered difficult. There is a need for a plating method which allows for control of the Ag ion content in solution, and thus control of the Ag metal content in the alloy solder wafer bump.
Another problem facing microelectronic device manufacturers using Sn—Ag alloy solder wafer bumps is low throughput due to the limited current densities which may be achieved using conventional electroplating baths. For example, in U.S. Pat. No. 6,638,847, it was reported that current densities appropriate for electroplating Sn-based alloys, include Sn—Ag, were in the range of 3-5 ASD. Kim et al. report current densities for plating a Sn—Ag solder using thiourea as a complexing agent in the range of 1 to 3 ASD. See Effects of Electroplating Parameters on Composition of Sn—Ag Solder, J. Electronic Materials, December 2004. Accordingly, there is a need for a plating composition which can plate at high current densities to achieve higher throughput.